Materials such as oxides, resins, glass, and metals usually demonstrate positive thermal expansion. By contrast, Evans et al. have recently reported various composite oxides demonstrating negative thermal expansion. For example, Evans et al. have conducted a comprehensive analysis of the mechanism of such negative thermal expansion in tungsten oxides such as ZrW2O8 and Sc(WO4)3 (U.S. Pat. Nos. 5,322,559, 5,433,778, 5,514,360, 5,919,720, and 6,183,716; J. S. O. Evans et al., Physica B, 241–243, pp. 311–316 (1998); J. S. O Evans et al., J. Solid. State Chem., 137, pp. 148–160 (1998)). In particular, a tungstic acid compound represented by formula A3+2(M6+O4)3 was indicated to demonstrate the relationship between the ion radius of the metal element A3+ and thermal expansion coefficient and is known to have a negative thermal expansion coefficient that tends to increase with the increase in the ion radius of the metal element (D. A. Woodcock et al., J. Solid State Chem, 149, pp. 92–98 (2000)). Further, composite systems in trivalent metal elements in the tungstic acid compound represented by formula A3+2 (M6+O4)3 were comprehensively studied and Al1.7In0.3(WO4)3 was disclosed to have a linear thermal expansion coefficient of 0.2×10−6 K−1, as measured by thermomechanical analysis (U.S. Pat. No. 5,919,720).
The inventors have studied mixing and firing, for example, of a HfW2O8 as a material having a negative thermal expansion coefficient and MgWO4 as a material having a positive thermal expansion coefficient and have shown that a very low linear thermal expansion coefficient of ±1.0×10−6 K−1 or less can be obtained and that in this system the thermal expansion coefficient can be controlled (Japanese Unexamined Patent Publication No. 2003-89572). With respect to this material system, a tungsten oxide represented by formula (HfMg) (WO4)3 was synthesized and found to be composed of a quasibinary system in which changes in thermal expansion are due to mixing of (HfMg) (WO4)3 with MgWO4 or HfW2O8. Further, it was confirmed that a tungsten oxide represented by formula (HfMg) (WO4)3 can be formed by mixing and firing HfW2O8 and MgWO4 and that this material demonstrates a low thermal expansion coefficient.
Among other oxides, materials of a cordierite system represented by NEXERA (Shin-Nippon Seitetsu K. K.) are known to have a low thermal expansion coefficient (Japanese Unexamined Patent Publication Nos. 2000-290064 and 2001-019540).
Various types of low-expansion glass have been developed for optical lenses and a number of types of glass with low thermal expansion have been introduced (Technological Materials of Sigma Koki K. K. “Optical Polishing Products for Lasers and Thin-film Products”. Chapter 1: Glass Materials).